The use of image sensors is known in numerous applications ranging from the general-consumer gadgets sector, to the professional photography, and to industrial, medical and/or scientific uses, just to cite a few.
A typical image sensor comprises a plurality of pixels, each comprising a photosensitive element or photodetector, that are operatively connected to a control unit that includes a readout circuit for selectively reading out the photo-signal generated by the light impinging on the photosensitive element of each pixel of the plurality of pixels.
Most image sensors use a photodiode as the photosensitive element in their pixels. Given that the quantum efficiency of typical photodiodes cannot exceed one for the visible and infrared ranges, such image sensors critically rely on reaching very low noise levels and/or on using long exposure times, to achieve high signal-to-noise ratios.
However, both of these techniques have important shortcomings. For example, designing the image sensor circuitry to achieve low noise requires placing a pre-amplification stage as close to the charge-generating element (i.e., the photodiode) as possible, as it is done for instance in an active pixel sensors in which the an amplifier is integrated inside the pixel. Moreover, the design of the overall readout circuit becomes more sophisticated. On the other hand, increasing the exposure time reduces the effective frame rate of the image sensor and may lead to blurring effects. Moreover, a longer exposure time enhances the adverse effect of thermal noise, which in turn makes the design requirements for the readout circuit even more demanding.
Other known technologies, such as for instance avalanche photodiodes or image intensifiers, despite being able to provide photodetectors with some photoconductive gain via carrier multiplication effects, have proven to be difficult to integrate into high-resolution image sensors. Moreover, these technologies require operation conditions that are unsuitable for practical image sensors (e.g. avalanche photodiodes typically require very high reverse bias voltages for proper operation), as described for example in chapter 2 of “Smart CMOS Image Sensors and Applications”, Jun Ohta, CRC Press, Sep. 19 2007.
The use of active devices based on two-dimensional (2D) materials, such as for instance graphene, for different applications is the object of on-going research. For example, single-pixel photodetectors having a photosensitive element made of graphene have been demonstrated as proof of concept. The use of photodetectors based on 2D materials (e.g., graphene, as disclosed in for instance U.S. Pat. No. 8,053,782 B2) or on semiconductor nanocrystals (e.g. quantum dots, see for example patent U.S. Pat. No. 8,803,128 B2) in the pixels of full-size image sensors has also been proposed. However, such image sensors typically exhibit limited photoconductive gain.
Document WO 2013/017605 A1 discloses a phototransistor comprising a transport layer made of graphene, and a sensitizing layer disposed above the transport layer and that is made of colloidal quantum dots. The sensitizing layer absorbs incident light and induces changes in the conductivity of the transport layer to which it is associated. The high carrier mobility of graphene and the long carrier lifetime in the quantum dots make it possible for the phototransistor disclosed therein to obtain a large photoconductive gain. However, the device can only achieve desired responsivity levels at the expense of increased dark current levels, which in turn degrade the sensitivity and the shot-noise limit of the device.
Therefore, it would be highly desirable to have image sensors in which the photosensitive element of their pixels was capable of providing a high photoconductive gain, but without compromising the pixel sensitivity due to, for example, high dark current levels.
Another important aspect to take into account is the spectral range in which an image sensor is to operate as it will greatly determine the choice of the available light-absorbing materials for the fabrication of the photosensitive element of the pixels.
In that sense, silicon is widely used in image sensors operating in the visible and near infrared ranges. In contrast, compounds such as InGaAs or HgCdTe, among others, are often employed for the infrared range (including short-wave infrared and/or long-wave infrared subranges). Finally, for image sensors operating in the ultraviolet region, and shorter-wave ranges, some known suitable materials include wide-gap semiconductors, such as for instance AlGaN. Alternatively, technologies based on back-thinning of silicon or on intensified imagers, such as for example microchannel plate (MCP) photodetectors, can also be used for shorter-wave ranges.
On the other hand, in most image sensors the readout circuit (usually also referred to as readout integrated circuit, or ROIC) is implemented in silicon, for example using CMOS technology.
This means that a monolithic integration of the plurality of pixels of an image sensor with the readout circuit of said pixels can only be achieved for those image sensors designed to operate in the visible and/or near infrared ranges. However, image sensors operating in other spectral ranges will require hybrid integration of silicon (e.g., CMOS technology) with other materials used for the photodetectors of the pixels, such as InGaAs. Such hybrid integration involves difficult and costly bonding processes, as described for example in US 2008/093554 A1 and in U.S. Pat. No. 6,107,618 A, which in turn impose a lower limit on the pixel size.
Developed in the last years, three dimensional (3D) integrated circuit technology allows the fabrication of integrated circuits by arranging active devices (e.g. transistors) in several levels at different heights, hence advantageously exploiting the third dimension of the structure.
In addition to obtaining very compact structures with reduced footprint, 3D integrated circuits offer an improved electrical performance compared to conventional integrated circuits. For example, as electrical interconnects can be distributed over an entire surface between levels of active devices, a higher density of shorter interconnects is possible, which results in faster circuits featuring more bandwidth. In addition, heterogeneous integration of circuits of different manufacturing technologies and/or materials becomes possible, by using for example wafer bumping processes to form interconnects.
A first type of 3D fabrication technology, known as 3D packaging, consists of stacking several semiconductor wafers and/or dies and interconnecting them vertically using through-substrate vias (TSVs) and traditional interconnect technology such as wire and/or flip-chip bonding to achieve a fully-operative vertical stack. Alternatively, monolithic 3D integration is another type of 3D fabrication technology in which layers of active devices are grown or deposited sequentially on a same substrate.
Document U.S. Pat. No. 8,796,741 B2 discloses a monolithic 3D integrated circuit device that includes a first level comprising a first plurality of active devices, and a second level comprising a second plurality of active devices that comprise a layer of graphene.
It is therefore an object of the present invention to provide an enhanced image sensor in which the integration of its pixels with the control unit can be done in a simple and efficient manner while leading to a highly compact integrated circuit architecture.
It is also an object of the present invention to provide an image sensor in which its pixels comprise an improved photosensitive element capable of high photoconductive gain, enhanced responsivity, and/or short response time.
It is yet another object of the present invention to provide an image sensor with an improved sensitivity of its pixels, and that does not require deep cooling of the device to achieve high signal-to-noise ratios.
US 2011/315949 A1 discloses an apparatus and method for sensing photons, where the apparatus includes a plurality of photon sensing layers arranged on top of each other, and an intermediate color filtering layer between each two adjacent photon sensing layers to prevent a respective color component of light from proceeding into the photon sensing layer next to it. Regarding the color filtering layers, for some embodiments, they include reflective coatings made of, for example, a ZnO layer. No other function for such a ZnO layer is disclosed in US 2011/315949.
To fully understand the differences between the apparatus of US 2011/315949 A1 and the present invention, it must be pointed out that a photo sensitizing layer (as defined in the present document) absorbs incident light and induces changes in the conductivity of the transport layer to which it is associated. The apparatus of US 2011/315949 A1 does not include such an arrangement of a photo sensitizing layer (i.e. an absorbing layer) and a transport layer, but, in contrast, the absorbing layer is the same as the transport layer, specifically the layer called photon sensing layer, and hence no photo sensitizing layer is disclosed at all in US 2011/315949 A1.
In the apparatus disclosed in US 2011/315949 A1, the photo sensing layer, which is made of graphene, absorbs only about 2.3% of the light and hence many layers of graphene are needed to achieve a high external quantum efficiency.
No dark current suppressing circuit is either disclosed in US 2011/315949 A1. Indeed, neither the electrodes nor any other element of the apparatus of US 2011/315949 A1 implements a dark current suppressing circuit.